The present invention relates to a resistor, and in particular, to a trimmed resistor of a thick film type used in various electric equipments and to a trimmed resistor used in printed circuit boards of a hybrid integrated circuit.
A trimmed resistor means a resistor whose resistance value is adjusted by shaving off or cutting off a part of a film-like resistance body.
FIGS. 1a is a plan view of a conventional trimmed resistor of a thick film type in an operational condition and FIG. 1b is a sectional view of the resistor taken along the line Ib--Ib of FIG. 1a. The trimmed resistor of a thick film type comprises external electrodes 2 at both ends of an insulation base board 1 and a thick film-like resistance body 3 disposed on the surface of the insulation board 1 between the electrodes 2 and covered by a protective film 4. In general, the insulation base board 1 is made from an alumina plate, the external electrodes 2 from a conductive material of palladium-silver group, the thick film-like resistance body 3 from a resistive material of ruthenium oxide group and the protective film 4 from a glass having a low melting point.
A method, in other words, a fabricating and mounting method, of the thick film-like trimmed resistor constructed as mentioned above will be described below. Referring to FIGS. 1a and 1b, on a circuit pattern formed on an upper surface of a circuit board 5, there are formed soft soldering lands 6 by using an usual material and by applying an usual method, correspondingly to the external electrodes 2 locating at the both ends of the trimmed resistor of a thick film type. To these soft soldering lands 6 are connected the external electrodes 2 of the above-mentioned trimmed resistor of a thick film type with soft solder 7 by applying an usual soft solding method such as reflowing, solder dipping or use of a soft soldering tool. The resistance value can be adjusted by successively forming a cut slit 8 at the central portion of the thick film-like resistance body 3 by using an usual means such as a laser, a sand blast, and a cutter.
FIGS. 2a and 2b show a printed resistance circuit of a trimming type having a conventional thick film-like resistance body. Numeral 11 denotes an insulation base plate which is mainly made of alumina or the like. Electrode terminals 12a and 12b are formed on a surface of the insulation base plate by printing and baking palladium-silver group material thereon, and a thick film-like resistance body 13 is formed by printing and baking ruthenium oxide group material for connecting the above-mentioned electrode terminals 12a and 12b with each other. Trimming work is conducted by forming a cut slit at the central portion of the thick film-like resistance body 13 by an usual means such as a laser, a sand blast, and a cutter. Thus, the resistance value of the printed resistance circuit to be trimmed can be adjusted by controlling the depth of the slit 14 of the thick film-like resistance body 13.
The above-mentioned trimmed resistor of a prior art includes, however, the following problems:
1) When the depth of the slit in the resistance body is excessively increased, an electric current concentration may be caused at the remaining portion apart from cutting, thereby deteriorating the stability of the resistance value during a long time and decreasing the electric power capacity. As a result, it is difficult to obtain a wide range variation of the resistance value.
2) When the resistance body is completely cut off by accident, the resistance value becomes infinitely great, thereby causing a breakdown or an abnormal operation of some other part in an inconvenient case of circuit constitution.